Simple method for producing superhydrophobic carbon nanotube array

ABSTRACT

Efficient methods for producing a superhydrophobic carbon nanotube (CNT) array are set forth. The methods comprise providing a vertically aligned CNT array and performing vacuum pyrolysis on the CNT array to produce a superhydrophobic CNT array. These methods have several advantages over the prior art, such as operational simplicity and efficiency.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/081,421, filed Apr. 6, 2011, which claims priority to U.S.Provisional Application No. 61/321,831, filed Apr. 7, 2010. The contentsof these priority documents and all other references disclosed hereinare incorporated in their entirety for all purposes.

BACKGROUND OF THE INVENTION

Wetting properties of materials have interested researchers for decades,due to their relevance to numerous applications. The wetting propertiesof a material are dictated by its surface chemistry (Emsley, J.,Chemical Society reviews, 9(1):91-124 (1980); Wenzel, R. N., Industrial& Engineering Chemistry, 28(8):988-994 (1936)) and its topographicstructure (Bhushan, B. et al., Philosophical transactions-Royal Society.Mathematical, Physical and engineering sciences, 367(1894):1631-1672(2009); Gao, L. and McCarthy, T Langmuir, 23(18):9125-9127 (2007); Gao,L. and McCarthy, T., Journal of the American Chemical Society,128(28):9052-9053 (2006); Krupenkin, T. et al., Langmuir,20(10):3824-3827 (2004)).

Many investigations have been conducted to understand the surfaceproperties of superhydrophobic materials. A superhydrophobic surface isextremely difficult to wet; it typically has a static contact anglehigher than 150° and a contact angle hysteresis less than 10°. Wang, S.and Jiang, L., Advanced materials, 19(21):3423-3424 (2007); Men, X. etal., Applied physics. A, Materials science & processing, 98(2):275-280(2010); Bhushan, B. et al., Philosophical transactions-Royal Society.Mathematical, Physical and engineering sciences, 367(1894):1631-1672(2009).

Superhydrophobic materials can be utilized as a protective coating forcreating a self-cleaning, nonstick surface (e.g., for solar panels) andfor preventing biofouling. Scardino, A. J. et al., Biofouling: TheJournal of Bioadhesion and Biofilm Research, 25(8):757-767 (2009). Theycan be used as electrodes to store charge energy in a non-aqueoussupercapacitor. They can also be employed to reduce hydrodynamic skinfriction drag in laminar and turbulent flow. Rothstein, J., AnnualReview of Fluid Mechanics, 42(1):89-109 (2010). Without intending to bebound by theory, the existence of a thin layer of trapped air at theliquid-solid interface is believed to allow a slip velocity at the wallof superhydrophobic material, reducing shear stress or momentum transferfrom the flow to the wall. Ou, J. et al. Physics of Fluids, 16:4635-4643(2004); MM, T.; Kim, J. Physics of Fluids 16:L55-L58 (2004); Daniello,R. J. et al. Physics of Fluids 21, online publ. no. 085103 (2009). Thiseffect can produce advantages at macro- or micro-scale. For example,superhydrophobic materials could reduce fuel consumption of marinevessels and the efficiency of liquid pipelines. They also could be usedin drug delivery devices to protect the device or drug from contact withblood, and they could be used to alter the mechanical response of cells.

In recent years, production of synthetic materials that exhibitsuperhydrophobic behavior has been reported. Among these materials,vertically aligned, multi-walled carbon nanotube arrays have gainedenormous attention, due to their simple fabrication process and inherenttwo-length scale topographic structure. Efforts have been made to modifythe surface chemistry of the carbon nanotube arrays so that theirwetting properties can be tuned precisely. The carbon nanotube arrayscan be made hydrophilic by functionalizing their surfaces withoxygenated surface functional groups that allow hydrogen bonds withwater molecules to form or hydrophobic by removing those oxygenatedsurface functional groups from their surfaces.

Various oxidation processes can be used to functionalize the surface ofcarbon nanotube arrays, such as high-temperature annealing in air,UV/ozone treatment, oxygen plasma treatment, and acid treatment.Processes like high-temperature annealing in air and oxygen plasmatreatment would be very costly to implement in large scale, not tomention highly probable to over-oxidize the carbon nanotube if anincorrect recipe were used. The acid treatment is generally hazardous,making it inconvenient to work with. On the other hand, the UV/ozonetreatment is a simple, safe, and cost-efficient method of producing morehydrophilic carbon nanotubes.

However, no analogous simple, safe, cost-efficient process has yet beenidentified for producing superhydrophobic carbon nanotubes. Previouslyreported studies suggest that complicated processes are always involvedin producing superhydrophobic carbon nanotube arrays. In order to makethese arrays superhydrophobic, they have to be coated with non-wettingchemicals such as poly(tetrafluoroethylene) (PTFE), zinc (II) oxide, andfluoroalkylsilane, (Huang, L. et al., The journal of physical chemistry,B, 109(16):7746-7748 (2005); Lau, K. et al., Nano Lett., 3(12):1701-1705(2003); Feng, L. et al., Advanced materials, 14(24):1857-1860 (2002)) orbe modified by plasma treatments, such as CF4, CH4, and NF3. (Hong, Y.and Uhm, H., Applied physics letters, 88(24):244101 (2006); Cho, S. etal., Journal of materials chemistry, 17(3):232-237 (2007)); Balu, B. etal. Langmuir, 24:4785-4790 (2008). However, no prior art has reported amethod for producing a superhydrophobic CNT array surface from pure CNTsgrown by a simple self-assembly process.

In view of the foregoing, there is a need for a simple, safe,cost-efficient process for producing superhydrophobic carbon nanotubes.Such a process could help to speed the investigation and the commercialapplication of superhydrophobic carbon nanotubes. The present inventionsatisfies these and other needs.

BRIEF SUMMARY OF THE INVENTION

In one embodiment, the present invention presents a method for producinga hydrophobic carbon nanotube (CNT) array, the method comprising:

providing a vertically aligned CNT array; and

performing vacuum-pyrolysis on the CNT array to produce the hydrophobicnanotube array. Preferably, the hydrophobic nanotube CNT array issuperhydrophobic (i.e., a superhydrophobic CNT array).

Preferably, the vacuum-pyrolysis step is performed under reducedpressure of about 0.5 torr to about 10 torr. More preferably, thevacuum-pyrolysis step is performed under reduced pressure of about 1torr to about 5 torr.

Preferably, the vacuum-pyrolysis step is performed at a reactiontemperature of about 100° C. to about 500° C. More preferably, thevacuum-pyrolysis step is performed at a reaction temperature of about125° C. to about 300° C.

Preferably, the vacuum-pyrolysis step has a duration of about one hourto about five hours.

Preferably, the vertically aligned CNT is anchored on a surface.Preferably, the vertically aligned CNT array is a member selected from asingle-wall CNT array, a multiwall CNT array, and a mixture of asingle-wall CNT array and a multiwall CNT array.

Preferably, the vertically aligned CNT array is synthesized using asynthesis technique that is selected from chemical vapor deposition(CVD), laser ablation, and arc discharge. Preferably, the verticallyaligned CNT is provided by a CVD process. In one aspect of theinvention, the CVD process is continuous with the vacuum-pyrolysis step.

Preferably, the method for producing a hydrophobic CNT array furthercomprises an oxidation step before the vacuum pyrolysis step to removeamorphous carbon.

Preferably, the method for producing a hydrophobic CNT array furthercomprises removing contamination using the vacuum-pyrolysis step.

Preferably, an outer surface of the superhydrophobic CNT array is atleast 85% free from oxygen-containing impurities. More preferably, theouter surface is at least 95% free from oxygen-containing impurities.

Preferably, the CNT array's static water droplet contact angle increasesbetween about 5% to 45% after the vacuum-pyrolysis step. Preferably, thewater droplet roll-off angle decreases by at least twofold. Preferably,more than one method is used to assess the array's superhydrophobicity(e.g., static water droplet contact angle and water droplet roll-offangle). Preferably, the static water droplet contact angle is betweenabout 160° to 180°. Preferably, the water droplet roll-off angle is fromabout 1° to 5°, which means that a water droplet would not maintain astable position on the surface of the array when the surface is tiltedmore than the roll-off angle.

Preferably, an outer surface of the superhydrophobic CNT array is atleast 85% free from oxygen-containing impurities. More preferably, theouter surface is at least 95% free from oxygen-containing impurities.Still more preferably, the outer surface is at least 97% free fromoxygen-containing impurities.

In another embodiment, the present invention presents a hydrophobic CNTarray, wherein the hydrophobic CNT array is produced by any of themethods claimed herein. Preferably, the hydrophobic CNT array issuperhydrophobic.

These and other aspects, objects and embodiments will become moreapparent when read with the detailed description and drawings thatfollow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. (a) Low-magnification scanning electron mircroscope (SEM) imageof vertically aligned carbon nanotube array. (b) High-magnification SEMimage of the same array showing the presence of some entanglements onthe array's top surface.

FIG. 2. (a) Water droplet on a superhydrophobic carbon nanotube arrayexhibiting an almost spherical shape with a 170° (±2°) static contactangle. (b) Time-lapse image of a water droplet bouncing off the surfaceof a superhydrophobic carbon nanotube array that was tilted 2.5°. Eachframe was taken with a 17 ms interval.

FIG. 3. Dispersion of carbon nanotubes with various wetting propertiesin industrial deionized (DI) water. The degree of CNT hydrophobicity isdecreasing from left to right. The four tubes from left to right are:the dispersion of superhydrophobic CNTs (contact angle about 170°);hydrophobic CNTs (contact angle about 143°); hydrophilic CNTs (contactangle about 75°); and strongly hydrophilic CNTs (contact angle about30°).

FIG. 4. A typical Fourier-transform infrared (FTIR) spectra fromsuperhydrophobic and hydrophilic carbon nanotube arrays showing strongpeaks at 810-1320 cm⁻¹, 1340-1600 cm⁻¹, 1650-1740 cm⁻¹, and 2800-3000cm⁻¹, which indicate the presence of C—O, C═C, C═O, and C-Hz stretchingmodes respectively.

FIG. 5. Electrochemical impedance modulus and phase-angle spectra ofcarbon nanotube arrays with various wetting properties in 1 M NaClaqueous solution. Superhydrophobic and hydrophilic arrays are indicatedby triangle and square markers respectively.

FIG. 6. A process diagram for one embodiment of the present method formaking superhydrophobic carbon nanotubes.

DETAILED DESCRIPTION OF THE INVENTION I. Definition of Terms

The terms “a,” “an,” or “the” as used herein not only includes aspectswith one member, but also includes aspects with more than one member.For example, an embodiment including “a vertically aligned CNT array”should be understood to present certain aspects with at least a secondvertically aligned CNT array.

The term “about” as used herein to modify a numerical value indicates adefined range around that value. If “X” were the value, “about X” wouldgenerally indicate a value from 0.95X to 1.05X. Any reference to “aboutX” specifically indicates at least the values X, 0.95X, 0.96X, 0.97X,0.98X, 0.99X, 1.01X, 1.02X, 1.03X, 1.04X, and 1.05X. Thus, “about X” isintended to teach and provide written description support for a claimlimitation of, e.g., “0.98X.” When the quantity “X” only includeswhole-integer values (e.g., “X carbons”), “about X” indicates from (X−1)to (X+1). In this case, “about X” as used herein specifically indicatesat least the values X, X−1, and X+1.

When “about” is applied to the beginning of a numerical range, itapplies to both ends of the range. Thus, “from about 5 to 45%” isequivalent to “from about 5% to about 45%.” When “about” is applied tothe first value of a set of values, it applies to all values in thatset. Thus, “about 7, 9, or 11%” is equivalent to “about 7%, about 9%, orabout 11%.”

A “hydrophobic” surface indicates a surface that is difficult to wetbecause of its chemical composition or geometric microstructure. Ahydrophobic surface has a static contact angle greater than 90°.

The term “or” as used herein should in general be construednon-exclusively. For example, an embodiment of “a composition comprisingA or B” would typically present an aspect with a composition comprisingboth A and B. “Or” should, however, be construed to exclude thoseaspects presented that cannot be combined without contradiction.

The term “outer surface of the carbon nanotube array” as used hereinincludes a side or face of an array that is not directly affixed to itssupport. Typically, the outer surface would be more likely to contactthe surrounding environment. For example, typical tests for roll-offangles would place the drop of liquid in contact with the outer surfaceof the array, not the inner surface, which would be the side of thearray affixed to the support.

A “superhydrophobic” surface indicates a surface that is extremelydifficult to wet because of its chemical composition or geometricmicrostructure. A superhydrophobic surface has at least one of thefollowing characteristics: a static contact angle greater than 150°, acontact angle hysteresis less than 10°, or a roll-off angle less than5°. Preferably, a superhydrophobic surface has two of thesecharacteristics; more preferably, all three characteristics.

II. Embodiments

In one embodiment, the present invention presents a method for producinga hydrophobic carbon nanotube (CNT) array, the method comprising:

providing a vertically aligned CNT array; and

performing vacuum pyrolysis on the vertically aligned CNT array toproduce the hydrophobic nanotube array. Preferably, the product CNTarray is a superhydrophobic CNT array.

In one aspect, the present invention provides a vacuum pyrolysis processto render carbon nanotube arrays superhydrophobic. Without being boundby theory, such processes are believed to reverse the effects ofoxidation by removing the oxygenated functional groups from the surfaceof the carbon nanotube, while maintaining the macroscopic structures andpacking density of the arrays. Therefore, no deposition of anynon-wetting foreign material (e.g., polyfluorocarbons such aspoly(tetrafluoroethylene); metal salts, such as zinc (II) oxide) on thearray is needed to make them superhydrophobic.

The temperature, pressure, and duration of the vacuum pyrolysis canaffect the process's efficiency. Typically, a vacuum pyrolysis processthat is performed at a moderate vacuum of 2.5 Torr and a temperature of250° C. for three hours is sufficient to completely deoxidize the array.

Preferably, the vacuum-pyrolysis step is performed under reducedpressure of about 0.5 torr to about 10 torr, such as 0.5, 0.6, 0.7, 0.8,0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 torr. More preferably, thevacuum-pyrolysis step is performed under reduced pressure of about 1torr to 5 torr or about 1 torr to 3 torr. Alternatively, thevacuum-pyrolysis step is performed under reduced pressure of about 1torr to 3 torr. In general, lower pressure is preferable. Withoutintending to be bound by theory, a lower pressure during the reactionfavors the oxygen-containing impurities' dissociation from the surface.Higher pressures disfavor the reaction, prolonging reaction times oreven preventing production of superhydrophobic CNT arrays.

At sufficiently low pressure, however, further decrease in pressureproduces only minor improvement in the reaction. For example, reactionpressures of 1 torr and 0.5 torr produces similar results in the vacuumpyrolysis (e.g., the process produced a similar superhydrophilic surfaceafter approximately the same total reaction time).

When oxygen is present in the ambient gas, however, it can oxidize thesurface of the starting CNT array, especially during pyrolysis at hightemperatures and relatively high pressures (e.g., >10 torr). Preferably,the pyrolysis is free from oxygen. Alternatively and more preferably,the pyrolysis is substantially free of oxygen, thereby avoidingoxidation of the superhydrophobic CNT surface.

Preferably, the vacuum-pyrolysis step is performed at a reactiontemperature of about 100° C. to about 500° C. such as 100° C., 150° C.,200° C., 250° C., 300° C., 350° C., 400° C., 450° C., or 500° C. Morepreferably, the vacuum-pyrolysis step is performed at a reactiontemperature of about 125° C. to 300° C. (e.g., about 250° C.). Lowtemperatures (e.g., <100° C., <75° C., or <50° C.) are disfavoredbecause they may not provide sufficient energy for the reaction toproceed efficiently. At high temperatures (e.g., >500° C., >575°C., >625° C., >700° C., >800° C., or >900° C.), the nanotubes or theirsupport (especially if the support is an organic polymer) may partiallyor completely decompose. Generally, higher temperatures produce a higherchance of decomposition and a faster rate of decomposition.

Preferably, the vacuum-pyrolysis step has a duration of about one hourto about five hours. In general, higher temperature and lower pressuresduring the pyrolysis step tend to decrease the time required to producea superhydrophobic CNT array. In some aspects, the vacuum pyrolysis canbe continuous. In some aspects, the vacuum pyrolysis includes one ormore periods of heating (e.g., two, three, or four heating cycles). Incertain aspects, the results of the procedure are dependent on the totalheating time rather than the number of heating cycles. In one preferredaspect, the present invention provides an iterative process in which thearray is subjected to vacuum pyrolysis, assayed for superhydrophobicity,and re-exposed to the vacuum-pyrolysis conditions if the array were notfound to be superhydrophobic.

CNT arrays are characterized by the orientation of the individualnanotubes composing the array. In a vertically aligned array, the axisrunning through the central point of a carbon nanotube's inner diameteris perpendicular to the array's base (i.e., if the nanotubes were pulledstraight out from their bases, they would be oriented like the teeth ofa comb or the hair on a head). This is in contrast to a horizontal array(e.g., like beads on a string) or a disordered array. Preferably, theCNT arrays of the present invention are vertically aligned arrays.Without intending to be bound by theory, this vertical alignmentminimizes each CNT's contact area with water, reducing possible van derWaals forces.

Preferably, the CNT array is anchored on a surface. Non-anchored tubescan be scraped off, which makes it harder for them to maintain theirsuperhydrophobic properties. Preferably, the CNT array is anchored to asilicon wafer base. Alternatively, the CNT array is anchored to apolymeric base (e.g., silicone). Sansom, E.; Rinderknect, D.; Gharib, M.Nanotech., 19, online publ. no. 035302 (2008). Procedures for makinganchored, aligned nanotubes and nanotube devices are known to theskilled artisan (e.g., U.S. Patent Application 2009/0130370; U.S. Pat.No. 7,491,628; U.S. Patent Application 2008/0145616; U.S. PatentApplication 2010/0196446; Han, Z. J. et al. Appl. Phys. Lett. 94, onlinepubl. no. 223106 (2009); Men, X.-H. et al. Appl. Phys. A, DOI10.1007/s00339-009-5425-6 (2009); Li, S. et al. J. Phys. Chem. B. 106,9274-9276 (2002); and Zhang, L. et al. Langmuir 25:4792-4798 (2009),which are incorporated by reference in their entirety).

Individual carbon nanotubes within the array can be single-wall ormultiwall. Single-wall nanotubes include one layer of carbon separatingthe inside and outside of the nanotube. The layer may include differentpatterns of carbon-carbon bonds depending on its two-dimensional bondgeometry. Multiwall nanotubes include more than one layer of carbonseparating the inside and outside. The multiple layers may be from asheet wrapping over itself or from separate, concentric nanotubes.Preferably, the vertically aligned CNT array is a member selected from asingle-wall CNT array, a multiwall CNT array, and a mixture of asingle-wall CNT array and a multiwall CNT array.

A CNT array is also characterized by the packing density of theindividual nanotubes composing the array. The packing density is thenumber of carbon nanotubes in an area; it is determined by the averagedistance between the different nanotubes in the array. In certainaspects of the present invention, a typical packing density is about 10⁶CNT/mm². At this packing density, the distance between nanotubes at thisdensity is about three to four times the diameter of the nanotube. Ahigher packing density is generally preferred because more closelyassociated nanotubes should make the array's surface more hydrophobic.

In certain preferred aspects, a major advantage of the present inventionis its ability to make even very short superhydrophobic CNT arrays.Previous studies have suggested that short CNT arrays cannot becomesuperhydrophobic. Lau, K. K. S. et al. Nano Lett. 3:1701-1705 (2009);Liu, H. et al. Soft Matter, 2:811-821 (2006). However, by usingvacuum-pyrolysis methods, CNT arrays can be made superhydrophobicregardless of length. For example, a CNT array as short as 10 μm can beconverted into a superhydrophobic array.

Preferably, the vertically aligned CNT array is synthesized using asynthesis technique that is selected from chemical vapor deposition(CVD), laser ablation, and arc discharge, using procedures commonlyknown to the skilled artisan. Preferably, the vertically aligned CNT isprovided by a CVD process (e.g., Seo, J. W. et al. New J. Physics, 5,120.1-120.22 (2003)).

Carbon nanotube arrays can also be prepared using other procedures knownto the skilled artisan, such as those set forth in U.S. Pat. No.7,491,628; U.S. Patent Application No. 2008/0145616; U.S. PatentApplication No. 2003/0180472; and U.S. Patent Application 2010/0247777.

In some aspects, the CVD process is continuous with (or at leastpartially continuous with) the vacuum-pyrolysis step. For example, ifthe CVD process is continuous with the vacuum-pyrolysis step, thevacuum-pyrolysis process can be merged with the CNT growth process toform a continuous process (e.g., if there is no need to anchor the CNTarray). During the cool-down from CVD synthesis of nanotubes, a vacuumis applied rather than a flowing inert gas. In some aspects, thismodification eliminates a need for inert gas purging.

Some CNT arrays can contain residual catalyst particles or amorphouscarbon, e.g., from the CNT synthesis. These impurities may createdefects in the array. In certain aspects, the process set forth in thepresent invention further comprises an oxidation step before thevacuum-pyrolysis step to remove amorphous carbon. Preferably, ifanalytical techniques indicated a significant amount of catalystparticle leftovers or amorphous carbon in the CNT array, the array couldbe treated with ozone to oxidize the impurities before the vacuumpyrolysis (e.g., by exposure to 185 nm UV radiation in air for 1 hr).

Various other oxidation processes can be used to remove catalystparticles leftovers or amorphous carbon other than the ozone treatment.These other processes include hot air annealing, oxygen plasmatreatment, and acid (usually a mixture of nitric acid and hydrochloricacid) treatment (e.g., Tohji, K. et al. Nature, 383:679 (1996)). Whilehot air annealing, oxygen plasma treatment, and acid treatment are eachmore effective in removing the catalyst particles leftovers andamorphous carbon than the ozone treatment, these processes are harsherso that the chance to over-oxidize the CNT array is high.

The easiest way to find catalyst particle leftovers and amorphous carbonis by performing electron microscopy analysis on the CNT samples;preferably, by using transmission electron microscopy (TEM) on the CNTsamples. If the catalyst particles are only found inside the CNTs and ifthe thickness of amorphous carbon is much less than the diameter of theCNTs, the preliminary oxidation is unnecessary.

In one aspect, a preliminary oxidation is performed if (i) there is anysign of more than one catalyst particle on the average (e.g.,preferably, the mean) found on the outer surface of each CNT or (ii) thethickness of amorphous carbon is more than the diameter of the CNT. Forexample, if TEM indicated 76 surface catalyst particles in a samplecomprising 75 nanotubes, the array would be oxidized, but if TEMindicated only 75 or fewer particles in the sample, the array would notbe oxidized. Alternatively, if the average number of outer surfacecatalyst particles is at least one, the array is oxidized to removethem. In some preferred aspects, about 25 to 250, about 50 to 200, orabout 60 to 200 CNTs are examined by TEM to make this determination.

Some CNT arrays can contain other impurities or contaminants that mayadversely affect the array's properties. These impurities may bevolatile or may decompose into volatile products under vacuum-pyrolysisconditions. In certain aspects, the process set forth in the presentinvention further comprises removing contamination using thevacuum-pyrolysis step.

Preferably, the vacuum-pyrolysis step removes oxygen-bearing impuritiesfrom an outer surface of the CNT array. More preferably, theoxygen-bearing impurities are organic (i.e., carbon-containing).Oxygen-bearing, organic impurities can include organic compoundscontaining hydroxyl, carbonyl (e.g., aldehyde or ketone), or carboxyl(e.g., carboxylic acid) groups. Alternatively, the impurities can beorganic, oxygen-bearing groups chemically bonded to the surface of theCNT array (e.g., a carboxy group with a covalent, carbon-carbon bondattaching it to a carbon nanotube).

Preferably, the water droplet roll-off angle decreases at leasttwo-fold; preferably, the angle decreases from two- to twenty-fold. Thisis the general assay for superhydrophobicity, but others can be used.Preferably, more than one method is used (e.g., static water dropletcontact angle and water droplet roll-off angle). Preferably, the staticwater droplet contact angle increases between about 5% to about 45%,such as 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, or 45% after thevacuum-pyrolysis step.

Preferably, the static water droplet contact angle is between about 160°to 180° (alternatively, the static water droplet contact angle is atleast 150°; preferably, at least 160°; and more preferably, at least170°). Surfaces with static water droplet contact angles of at least160° are extremely hydrophobic, making them particularly useful (i.e.,they are not subject to the “petal effect” allowing water to be pinnedto the surface). Preferably, the water droplet roll-off angle is fromabout 1° to 5°, such as about 1°, 2°, 3°, 4°, or 5°; more preferably,the roll-off angle is from about 1° to 3° (e.g., about 1°). Preferably,the contact angle hysteresis is at most 10°, such as between about 1° to10° (e.g., about 1°, 2°, 3°, 4°, 5°, 6°, 7°, 8°, 9°, or 10°); morepreferably, the contact angle hysteresis is at most 5°.

The outer surface of the CNT array can also be monitored foroxygen-bearing bonds as a way to identify the method's progress. Suchmonitoring can be carried out with conventional methods (e.g.,quantitative FTIR). Preferably, an outer surface of the superhydrophobicCNT array is at least 85% free from oxygen-containing impurities. Morepreferably, the outer surface is at least 95% free fromoxygen-containing impurities. Still more preferably, the outer surfaceis at least 97% free from oxygen-containing impurities. Alternatively,the outer surface can be free from oxygen-containing impurities to theinstrument's effective limit of detection.

In another embodiment, the present invention presents a superhydrophobicCNT array, wherein the hydrophobic CNT array is produced by any of themethods claimed herein. Preferably, the hydrophobic CNT array issuperhydrophobic.

In certain preferred aspects, a major advantage of the present inventionis the use of a simple, high-yielding procedure (vacuum pyrolysis) toproduce superhydrophobic CNT arrays. Known methods of generatingsuperhydrophobic CNT arrays are generally low-yielding, may involvecorrosive reagents (e.g., the corrosive gases used in plasma treatment),and may change other properties of the CNT array's surface (e.g.,treatment with metal oxide, which makes a continuous metal oxidesurface). The present invention presents an alternative method forgenerating superhydrophobic arrays that is simpler and more efficient.In addition, it better preserves the microstructure of the CNT array.

In certain preferred aspects, another advantage of the present inventionis the effects of the removal of oxygen-containing impurities from theCNT array's outer surface to produce superhydrophobic CNT arrays. Knownmethods of generating superhydrophobic CNT arrays are generallylow-yielding, may involve corrosive reagents (e.g., the corrosive gasesused in plasma treatment), and may change other properties of the CNTarray's surface (e.g., treatment with metal oxide, which makes acontinuous metal oxide surface). The present invention's removal ofoxygen-containing impurities is a simpler and more efficient method ofproducing superhydrophobic arrays. In addition, it better preserves themicrostructure of the CNT array.

III. Examples

It is understood that the examples and embodiments described herein arefor illustrative purposes only. Various modifications or changes thereofwill be suggested to persons skilled in the art, and they are to beincluded within the purview of this application and the scope of theappended claims. In addition, each reference provided herein isincorporated by reference in its entirety to the same extent as if eachreference was individually incorporated by reference.

Example 1 Preparation of Superhydrophobic CNT Arrays

Carbon nanotube arrays used in this study were grown by the standardchemical vapor deposition (CVD) technique on a silicon substrate, usinghydrogen and ethylene as the precursor gas. Sansom, E. et al.,Nanotechnology, 19(3):035302 (2008). The average length of all thearrays was chosen to be about 14±4 μm (FIG. 1 a), which was about theminimum length that can be made using CVD techniques while preservingthe overall vertical alignment and high packing density of the arrays(FIG. 1 b). The main reason this length was chosen is for thedifficulties in producing a superhydrophobic surface out of short carbonnanotube arrays reported in the previously reported studies. Lau, K. etal., Nano letters, 3(12):1701-1705 (2003).

The CNT arrays were subjected to vacuum pyrolysis, typically at a vacuumof 2.5 Torr and a temperature of 250° C. for three hours. After thepyrolysis, the array's static contact angle was tested by conventionalmethods to determine its hydrophobicity. If conventional analyticalmethods indicated that the array was not superhydrophobic (e.g., if thestatic contact angle were less than 160°), the array was re-subjected tovacuum pyrolysis for another three hours (or longer if re-analysis afterthe second pyrolysis indicated that the array was still notsuperhydrophobic).

After being subjected to the vacuum-pyrolysis process, the carbonnanotube arrays exhibited extreme water repellency. Theirsuperhydrophobicity was demonstrated by their ultra-high static contactangle of 170° (±2°) (FIG. 2 a) and very low contact angle hysteresis of3° (±1°). These arrays also exhibit a very low roll-off angle of 1° (cf.FIG. 2 b, though FIG. 2 b shows a roll-off angle of 2.5°). The staticcontact angle, contact angle hysteresis, and roll-off angles weremeasured using standard techniques known by the skilled artisan (e.g.,contact angles were measured with a contact angle goniometer).

Example 2 Comparison of Post-Vacuum-Pyrolysis CNTs with Control CNTs

Comparison of water-based dispersions of the pre- and post-vacuumpyrolysis carbon nanotubes provides further evidence of the vacuumpyrolysis products' superhydrophobicity. Superhydrophobic CNT arrayswere prepared by the method of Example 1. These were compared withnon-superhydrophobic control arrays prepared by the same initialprocedure, but not subjected to vacuum pyrolysis (contact angle about143°) as well as hydrophilic CNTs (contact angle about 75°); andstrongly hydrophilic CNTs (contact angle about 30°). The water-baseddispersions are obtained by scraping the nanotube arrays from theirgrowth substrates and ultrasonically dispersing them in standardindustrial deionized water for at least two hours.

The experiment demonstrated that nanotubes that have been subjected tovacuum-pyrolysis were not dispersed in water even after being sonicatedfor more than two hours (FIG. 3). In contrast, the more hydrophilicnanotubes can be dispersed easily in water. From this finding, one canconclude that the vacuum-pyrolysis treatment is capable of completelydeoxidizing individual nanotubes within the array.

Example 3 FTIR and Electrochemical Characterization of SuperhydrophobicCNT Surface Chemistry

To study the effect of the vacuum-pyrolysis process on the surfacechemistry of the hydrophilic CNTs, FTIR spectrometry analysis wasconducted on array samples using standard methods for the skilledartisan. The superhydrophobic samples were compared with hydrophilicsamples (contact angle 30°, as per Example 2's strongly hydrophilicCNTs). A small portion of the CNT array (<1 mm²) was scraped from thegrowth substrate, dispersed in 50 ml deuterated dichloromethane,drop-cast onto a KBr window, and then dried overnight under mild vacuum(>5 torr) and without heating to remove the solvent. The FTIRspectrometry analysis was subsequently performed on the sample using aninfrared laser with a wavelength of 2500-12500 nm.

Four strong bands were detected on the hydrophilic arrays at 810-1320cm⁻¹, 1340-1600 cm⁻¹, 1650-1740 cm⁻¹, and 2800-3000 cm⁻¹, which indicatethe presence of C—O, C═C, C═O and C—H_(x) stretching modes respectively(FIG. 4). The peaks at 970, 1028, 1154 and 1201 cm⁻¹ correspond to C—Ostretching modes (Kuznetsova, A. et al., Chemical Physics Letters,321(3-4):292-296 (2000)), and the broad shoulder band at 810-1320 cm⁻¹suggests the existence of C—O—C bonds from ester functional groups.Sham, M. and Kim, J., Carbon, 44(4):768-777 (2006); Socrates, G.,Infrared and Raman characteristic group frequencies: tables and charts,3rd ed. ed., Wiley: Chichester (2001); Mawhinney, D. et al., Journal ofthe American Chemical Society, 122(10):2383-2384 (2000); Kim, U. et al.,Physical Review Letters, 95(15):157402 (2005). The peaks at 1378, 1462,1541 and 1574 cm⁻¹ indicate the presence of C═C stretching vibrationmodes of the carbon nanotube walls. Kuznetsova, A. et al., ChemicalPhysics Letters, 321(3-4):292-296 (2000); Sham, M. and Kim, J., Carbon,44(4):768-777 (2006); Socrates, G., Infrared and Raman characteristicgroup frequencies: tables and charts, 3rd ed. ed., Wiley: Chichester(2001); Mawhinney, D. et al., Journal of the American Chemical Society,122(10):2383-2384 (2000). The narrow band at a peak of 1703 cm⁻¹corresponds to C═O stretching modes of either quinone or carboxylic acidester groups. Kuznetsova, A. et al., Chemical Physics Letters,321(3-4):292-296 (2000); Sham, M. and Kim, J., Carbon, 44(4):768-777(2006); Mawhinney, D. et al., Journal of the American Chemical Society,122(10):2383-2384 (2000); Kim, U. et al., Physical Review Letters,95(15):157402 (2005).

These FTIR spectra show that the strength of all peaks associated withthe C—O and C═O stretching modes of the superhydrophobic array issignificantly lower than that of the hydrophilic one, suggesting thatthe oxygen desorption process does take place during vacuum-pyrolysistreatment. The strength of the C═C stretching modes also seems todecrease slightly, implying that the graphitic structures of the carbonnanotubes were still intact after the vacuum-pyrolysis treatment. Thetriplet with peaks at 2848, 2915 and 2956 cm⁻¹ indicate C—H_(x) bondsfrom the hydrocarbon functional group. Kim, U. et al., Physical ReviewLetters, 95(15):157402 (2005). This hydrocarbon triplet peaks seems tobe unaffected by vacuum-pyrolysis process, implying that these peaks maybe associated with contaminations in the FTIR instrument (Kim, U. etal., Physical Review Letters, 95(15):157402 (2005)) and have nothing todo with the wetting properties of the arrays.

Just like their wetting properties, the electrochemical properties ofcarbon nanotube arrays are dictated by their surface chemistry. As shownby the measured impedance modulus and phase angle spectra, carbonnanotube arrays with different wetting properties exhibit differentelectrochemical properties (FIG. 5). For the superhydrophobic array, thefrequency of constant impedance spans for three decades from 1 kHz to 1MHz. On the other hand, the frequency of constant impedance for thehydrophilic arrays spans for six decades from 1 Hz to 1 MHz. At a lowfrequency of 10 mHz, the impedance modulus of the superhydrophobic arrayis about two orders of magnitude higher than that of the hydrophilicone. The impedance of the hydrophilic and the superhydrophilic CNTarrays were found to be about 650Ω and 162 kΩ respectively at frequencyof 12 mHz in 1 M NaCl solution. This finding implies that the specificcapacitance for hydrophilic and the superhydrophilic CNT array is about3.3 F/g and 9.1 mF/g respectively.

Without being bound by theory, these findings are the result of a thinfilm of air on the interface between the surface of the superhydrophobicarray and the aqueous electrolyte. This air film inhibits electronstransfer from the arrays and obstructs protons in the electrolyte toapproach the surface of the array. On the other hand, the hydrophilicarray is completely wetted by the aqueous electrolyte such that there isno air film that may inhibit electron transfer from the arrays. Becauseof the air film's presence film, the impedance of the superhydrophobicarray was measured to be two orders of magnitude higher than that of thehydrophilic one.

Example 4 Flow-Diagram of One Embodiment of the Present Invention

This example illustrates a flow diagram of one embodiment of the presentinvention (FIG. 6). The embodiment provides a vacuum pyrolysis process(100) to render carbon nanotube arrays superhydrophobic. In thisinstance, beginning with a vertically aligned CNT array (110), the arrayis analyzed for any catalyst particles or amorphous carbon contamination(117). If either or both of these are present, an oxidation process isperformed to remove the contamination (121). Next, a vacuum-pyrolysisstep is performed at a reaction temperature and duration as indicatedherein (e.g., a temperature selected from about 100° C. to about 500° C.and a duration selected from about one hour to five hours) (125). Afterthe vacuum-pyrolysis step is performed, the static contact angle isdetermined. In certain embodiments, if the static contact angle iswithin specification (136), the roll-off angle is determined. In oneaspect, if the roll-off angle is within specification (147), thesuperhydrophobic CNT array is produced (163). If either of the staticangle or the roll-off angle are not within specification, thevacuum-pyrolysis step (125) may be performed iteratively to produce thesuperhydrophobic CNT array (163).

CONCLUSIONS

In conclusion, the discoveries reported herein show that the wettingproperties of carbon nanotube arrays can be altered by controlling theamount of oxygenated functional groups that are bonded to their surface.The CNT arrays can be made hydrophilic by oxidizing with, e.g., hot air,strong acids, UV/ozone, or oxygen plasma. The CNT arrays can be madesuperhydrophobic by deoxidizing with vacuum-pyrolysis treatment atmoderate vacuum and temperature. Such vacuum-pyrolysis treatment iscapable of removing the oxygenated functional groups that are attachedto the CNTs' surfaces.

All publications and patent applications cited in this specification areherein incorporated by reference as if each individual publication orpatent application were specifically and individually indicated to beincorporated by reference. Although the foregoing invention has beendescribed in some detail by way of illustration and example for purposesof clarity of understanding, it will be readily apparent to those ofordinary skill in the art in light of the teachings of this inventionthat certain changes and modifications may be made thereto withoutdeparting from the spirit or scope of the appended claims.

What is claimed is:
 1. A superhydrophobic carbon nanotube (CNT) arraycomprising: a plurality of pyrolized vertically aligned carbon nanotubes(CNTs) on a substrate surface, wherein the CNTs are withoutcontamination by either catalyst particles or amorphous carbon, andwherein the CNTs are at least 85% free from oxygen-containingimpurities.
 2. The array of claim 1, wherein an outer surface of the CNTarray is at least 95% free from oxygen-containing impurities.
 3. Thearray of claim 1, wherein the plurality of CNTs are anchored to thesurface.
 4. The array of claim 1, wherein the plurality of CNTs areselected from the group consisting of: single-wall CNTs, multiwall CNTs,and a mixture of single-wall and multiwall CNTs.